The present invention relates to electric power circuit interrupters and, more particularly, relates to a circuit interrupter with limited restrike capability suitable for use as a line capacitor and load switch. The interrupter, which is disconnected from the circuit in normal operation, includes a bidirectional freewheeling toggle mechanism and a bellows around a relatively slow moving actuator shaft to minimize wear and tear imposed on the interrupter through repetitive cycles.
A circuit interrupter is a disconnect switch used to periodically disconnect and reconnect an electrical power transmission, sub-transmission or distribution line from a connected device, such as a load, a line capacitor, a voltage regulator, or another type of device. Circuit interrupters typically include two or more contacts that are in physical contact with one another when the electric power line is connected to the switched device, and that are physically separated when the line is disconnected from the switched device. The interrupter is said to be in the xe2x80x9cclosed positionxe2x80x9d when the contacts are in contact with each other, and in the xe2x80x9copen positionxe2x80x9d when the contacts are separated.
For an electric power line that carries high voltage and/or high current, it is desirable to open the male and female contacts quickly to avoid a restrike, in which the electric current arcs across a physical gap between the contacts. Restrikes impose high current spikes and serious voltage disturbances on the power line, and can also physically degrade the components of the interrupter, especially the contacts. These current spikes and voltage disturbances can also damage other pieces of equipment connected to the power line. Sensitive loads, such as computers and other electronic devices, are particularly vulnerable to this type of damage. Generally, the wider the arc gap during a restrike, the higher the voltage required to breakdown the gap, and the larger the current spike and the associated risk of damage.
Restrikes occur when the interrupter""s contacts are not in physical contact, but are still close enough to each other to permit electric current to arc through the air or other media between the contacts. When the contacts of a properly designed circuit interrupter are fully separated, the distance between the contacts is sufficient to prohibit restrikes. However, a restrike can occur as the contacts are moved through an xe2x80x9copening strokexe2x80x9d from the fully connected or closed position to the fully separated position or open position. Likewise, an arc can occur across a gap between the contacts as the contacts are moved through a xe2x80x9cclosing strokexe2x80x9d from the open position to the closed position. However, arcs during a closing stroke are less dangerous to the electric system because the current in the circuit is zero prior to reconnection, which greatly reduces the current spike caused by the arc. Nevertheless, for safety reasons it may desirable to control the arcs during reconnection of the circuit interrupter.
Restrikes typically occur because once the circuit is opened at a zero voltage crossing, there is a rapid rise in voltage across the contacts known as the xe2x80x9ctransient recovery voltage.xe2x80x9d If the interrupter""s contacts are not separated quickly enough for the gap between the contacts (the xe2x80x9carc gapxe2x80x9d) to withstand this rising voltage, then the gap breaks down and the current flow arcs across the gap and results in a restrike. A first restrike generally occurs at or near the point when the transient recovery voltage reaches its maximum value, which is typically one-quarter of a cycle from the zero voltage crossing when the circuit was initially opened. Thus, to prevent a restrike, the contacts must be moved from the closed position to a position at which a restrike is impossible within one-quarter of a cycle.
On an opening stroke in which the arc gap increases quickly, a second restrike is much more severe than the first because the arc gap is much larger. For this reason, in certain applications a maximum of one restrike is permitted. To meet this one-restrike-maximum, the contacts must be moved from the fully connected position to a position at which a restrike is impossible within three-quarters of a cycle. In particular, governmental regulations and municipal codes generally permit a maximum of one restrike per transmission or distribution line disconnection. Thus, the actuator mechanism of a typical interrupter must be capable of opening the contacts at a separation velocity sufficient to prevent multiple restrike (i.e., more than one) once the initial arc extinction occurs at a current zero.
Usually, a human operator of an interrupter cannot create enough energy to separate the contacts of an interrupter in a short enough time without a mechanical advantage. Thus, separation velocity is typically provided by an actuator mechanism, usually a spring arrangement, in the circuit interrupter. A typical spring arrangement stores potential energy in a spring-type actuator mechanism and then releases the spring energy abruptly to produce the desired separation velocity. Of course, higher separation velocity can often be accomplished by a more robust actuator mechanism. However, the designer of the circuit interrupter is also concerned with the cost and durability of the resulting device.
The designer therefore takes the intended use of the circuit interrupter into account when designing the circuit interrupter. For example, disconnection is often required to perform maintenance on the electrical power line or on a device connected to the line downstream from the disconnect switch, such as a transformer or voltage regulator. A disconnect capability may also be required for fault protection. A conventional circuit breaker is typically used as the circuit interrupter for these applications. In this application, the circuit breaker can be expected to cycle several dozen or a hundred or so times over its life span.
Line capacitor switches, on the other hand, can be expected to cycle much more frequently. This is because a line capacitor is typically switched into connection with the electric power line to correct the power factor during high-load periods. The line capacitor is later switched out of the circuit when the load drops and the power factor correction afforded by the capacitor is no longer needed. Because electric power loads typically peak on a daily or twice-daily basis, capacitor switches typically cycle on a daily or twice-daily basis. In addition, certain types of industrial loads, such as coal mines and arc furnaces, often impose peak loads many times each day. Therefore, a capacitor switch can be expected to cycle hundreds or thousands of times over its life span. A load switch, which is typically used to disconnect a discrete distribution voltage load such as customer-owned device or premises, may also experience hundreds or thousands of cycles over a lifetime.
In addition, it is economically feasible to design very expensive transmission voltage circuit breakers to provide fault protection for the transformer, which is a very expensive device. In addition, multiple restrikes at very high voltages can damage the transformer and other connected devices. Transmission voltage circuit breakers have therefore been designed with very robust actuator mechanisms, xe2x80x9cpenetrating contactsxe2x80x9d (e.g., a male xe2x80x9cpinxe2x80x9d contact and a female xe2x80x9ctulipxe2x80x9d contact) that fit into each other, sealed chambers that surround the penetrating contacts with a dielectric gas that quenches the arcs within xe2x80x9carc gapsxe2x80x9d between the contacts, and nozzles that direct the dielectric gas into the arc gaps as the penetrating contacts separate. Although these features are very effective at minimizing restrikes, they have traditionally been too expensive to be feasible for inclusion in sub-transmission and distribution voltage devices, such as capacitor and load switches.
Conventional circuit breakers have a number of other attributes that make them unsuitable as capacitor or load switches. First and most importantly, circuit breakers are not designed to withstand the hundreds or thousands of cycles that capacitor and load switches must withstand. For example, circuit breakers typically include xe2x80x9cstopxe2x80x9d mechanisms for charging and then abruptly releasing spring energy. These stop mechanisms are prone to wear and tear and thus limit the durability of the circuit breaker. Bellows placed around high-speed actuators to seal the dielectric gas chambers are also prone to wear and tear through repetitive cycling of the breaker. A circuit breaker would therefore break down far to quickly to be cost effective if used as a capacitor switch. Second, circuit breakers are normally operated as series-connected devices, which raises their cost as compared to disconnect switches that are normally disconnected from the circuit and only conduct current when temporarily connected during disconnect operations. Third, circuit breakers typically include separate actuator mechanisms for opening and closing the breaker, which also raises their cost as compared to a disconnect switch that includes a single actuator mechanism.
Electric switchgear manufacturers have developed circuit interrupters for sub-transmission and distribution applications that overcome some of these disadvantages. For example, normally disconnected circuit interrupters have been developed for use as capacitor and load switches. However, these devices are not designed to prevent restrikes, but instead include a series connected cascade of sacrificial xe2x80x9cbuttxe2x80x9d contacts that are designed to deteriorate over time as a result of restrikes. The deterioration of the contacts requires regular maintenance to monitor and replace the contacts as they deteriorate, and thus increases the cost of using this type of circuit interrupter. These devices are also prone to cascading failures when one of the butt contacts deteriorates to the point of malfunction. These circuit interrupters are also designed to control the arc only on the opening stroke, and typically conduct an uncontrolled arc through air on the closing stroke.
Although transmission voltage circuit breakers are available with penetrating contacts, dielectric gas chambers, and actuators that accelerate the penetrating contacts to quench arcs between the contacts within the dielectric chambers during circuit opening, these features are not presently available in sub-transmission or distribution devices, such as capacitor and load switches. Moreover, circuit breakers with these features are not presently designed to be economical enough to serve as capacitor or load switches. Available capacitor and load switches, on the other hand, are not presently designed to avoid multiple restrikes or to accelerate their contacts to control the resulting arcs on both the opening and closing strokes. The limited durability of conventional capacitor switches with sacrificial contacts also limits their feasibility for many applications.
Therefore, there is a need for a circuit interrupter that prevents or limits restrikes, and that is durable enough to be used as a capacitor and load switch. There is a further need for a normally disconnected capacitor switch that controls the arc on both the opening and closing strokes. There is also a need for more durable and cost effective capacitor and load switch designs.
The circuit interrupter of the present invention meets these needs in circuit interrupter that includes many of the features of conventional circuit breakers, including a plunging contactor, a dielectric gas chamber, and an actuator mechanism that accelerates the plunging contactor during circuit opening. Unlike conventional circuit breakers, however, the circuit interrupter of the present invention includes these features in a normally disconnected device that opens and closes the circuit in response to physical movement of a conventional disconnect switch blade arm. These attributes allow the circuit interrupter to operate as a normally disconnected sub-transmission or distribution voltage disconnect switch.
In addition, the circuit interrupter includes a number of features that improve its operation over conventional circuit breakers or disconnect switches. These features improve the durability of the circuit interrupter and allow it to quench arcs within the dielectric gas chamber on both opening and closing strokes, which improves the operation of the device as a capacitor and load switch. In particular, the circuit interrupter includes a bidirectional freewheeling toggle mechanism that stores and then abruptly releases spring energy to accelerate the plunging contactor on both the opening and closing strokes. This allows the circuit interrupter to quench arch within the dielectric gas chamber on only the opening stroke, or on both the opening and closing strokes. This improves the safety of the circuit interrupter while allowing the device to avoid multiple restrikes on only the opening stroke, or on both the opening and closing strokes.
The freewheeling toggle mechanism improves the durability of the circuit interrupter as compared to conventional designs with stops that allow a spring to store and then release spring energy. The circuit interrupter also includes a bellows to seal the dielectric gas chamber around a relatively slow moving actuator shaft to minimize wear and tear imposed on the interrupter through repetitive cycles. The circuit interrupter may also be positioned so that the actuator arm meets the spacing requirements of electric codes, which allows the blade arm of a conventional disconnect switch to trigger the circuit interrupter on both the opening and closing strokes. These characteristics make the circuit interrupter particularly well suited to operation as a capacitor or load switch.
The circuit interrupter may also include a voltage-clamping device, such as a metal-oxide varistor, connected in parallel across the contacts of the interrupter. The xe2x80x9cbreak downxe2x80x9d or xe2x80x9ctripxe2x80x9d voltage for the voltage-clamping device is typically set at or near one per-unit (i.e., the maximum system voltage), which causes the voltage-clamping device to conduct electricity whenever the voltage across the interrupter exceeds the maximum system voltage. In this configuration, the parallel-connected voltage-clamping device may operate to discharge a capacitive load switched by the circuit interrupter. In addition, by limiting the voltage across the circuit interrupter, the parallel-connected voltage-clamping device prevents restrikes from occurring within the circuit interrupter when the voltage across the interrupter during operation would otherwise exceed the no-restrike design voltage of the interrupter. For example, the parallel-connected voltage-clamping device may prevent restrikes from occurring within the circuit interrupter during capacitor switching, when the voltage across the interrupter would approach two per-unit (i.e., double the maximum system voltage) if the voltage-clamping device was not present, and the two per-unit voltage level exceeds the no-restrike design voltage of the interrupter.
Generally described, the invention may be employed as an interrupter for an electric power circuit. A plunging contactor having first and second contacts moves in an opening stroke from a closed position to an open position to electrically open the circuit, and in a closing stroke from the open position to the closed position to reset the interrupter. A bidirectional freewheeling toggle mechanism stores and abruptly releasing spring energy to accelerate movement of the plunging contactor in both the opening and closing strokes. In addition, an actuator arm moves the toggle mechanism and thereby causes the toggle mechanism to store and then abruptly release the spring energy in both the opening and closing strokes. The freewheeling toggle mechanism may include a single spring that drives the toggle mechanism in both the opening and closing strokes.
The interrupter may also include a sealed interrupter chamber filled with a dielectric gas, such as sulphur-hexaflouride (SF6) gas. In this case, the plunging contactor is located within the dielectric gas chamber and a piston forces a flow of the dielectric gas into an arc gap defined by a separation between the first and second contacts on both the opening and closing strokes. The gas flow is enhanced by a nozzle that directs the flow into the arc gap at a predetermined distance from the first or second contact, such as 1.5 inches. In particular, the toggle mechanism typically accelerates the plunging contactor to a separation velocity of at least about 100 inches per second when then arc gap reaches 1.5 inches during the opening stroke. On the closing stroke, the toggle mechanism accelerates the plunging contactor to a reconnection velocity of at least about 80 inches per second when then arc gap reaches 1.5 inches.
When the interrupter operates as a disconnect switch, the actuator arm is positioned to be movable from an initial position (i.e., lowered in a typical disconnect switch configuration) to an opposing position (i.e., raised in a typical disconnect switch configuration) by a conventional disconnect switch blade arm as the blade arm moves from a closed position (i.e., lowered in a typical disconnect switch configuration) to an open position (i.e., raised in a typical disconnect switch configuration) to trigger the opening stroke of the plunging contactor. When the blade arm is in the closed position, it electrically connects to a jaws to provide a first electric path for the circuit path.
During a first portion of the movement from the closed position to the open position and before electrically disconnecting from the jaws, the blade arm electrically connects to the actuator arm, which is electrically connected to the plunging contactor, to provide a second electric path for the circuit through the plunging contactor in parallel with the first electric path through the jaws. Then, during a second portion of the movement from the closed position to the open position, the blade arm electrically disconnects from the jaws and remains in electrical connection with the actuator arm to connect a series electrical path for circuit through the plunging contact.
In addition, the toggle mechanism is configured, before accelerating the plunging contactor to open the circuit during the opening stroke, to allow the blade arm to move through a sufficient distance to prevent the circuit from arcing between the blade arm and the jaws in response to separation of the first and second contacts. This causes an arc to be drawn and extinguished between the first and second contacts within the sealed interrupter chamber during the opening stroke. In one alternative, after completion of the opening stroke and upon reaching the opposing position, a counter weight connected to the actuator arm causes the actuator arm to automatically return to its initial position. This causes the plunging contactor to moved through the closing stroke to reset the interrupter.
In another alternative, after completion of the opening stroke and before the blade arm reaches the open position, the actuator arm passes through the opposing position, separates from the blade arm, returns to the opposing position, and temporarily remains substantially in the opposing position. Then, as the blade arm subsequently moves from the open position to the closed position, the blade arm electrically connects with and moves the actuator arm from the opposing position to the initial position and thereby triggers the penetrating contact to move through the closing stroke. In this case, the toggle mechanism is configured to accelerate the plunging contactor to close the circuit during the closing stroke before the blade arm to moves to a position that would allow the circuit to arc between the blade arm and the jaws. This causes an arc to be drawn and extinguished between the first and second contacts within the sealed interrupter chamber during the closing stroke.
The blade arm typically pivots about a base during movement between the open and closed positions, and includes a contact area for contacting the jaws when the blade arm is in the closed position. To meet electrical code requirements, the actuator arm is positioned in the opposing position such that the minimum distance between the contact area of the blade arm and the actuator arm is at least as great as the minimum distance between the contact area and the base of the blade arm. In other words, the distance between the actuator arm and the blade arm is at least as great as the distance between the blade arm and the jaws when the blade arm is in the open position (i.e., raised in a typical disconnect switch configuration) and the actuator arm is in the opposing position (i.e., raised in a typical disconnect switch configuration).
In order to provide the required xe2x80x9cdwellxe2x80x9d to allow the actuator arm to trigger as desired on other the opening and closing strokes, the toggle mechanism includes a cam surface positioned between the actuator arm and a linkage mechanically coupling the actuator arm to the plunging contactor by way of the toggle mechanism. The cam surface causes the toggle mechanism to trigger the opening stroke of the plunging contactor as the blade arm moves the actuator arm from the initial position to the opposing position, and also triggers the closing stroke of the plunging contactor as the blade arm moves the actuator arm from the opposing position to the initial position, while maintaining a sufficient distance between the blade arm and the jaws to prevent the circuit from arcing between the blade arm and the jaws.
In yet another alternative, the circuit interrupter includes a voltage-clamping device connected in parallel across the contacts of the interrupter. The voltage-clamping device has a voltage-level threshold that may be selected to prevent a restrike from occurring across the contacts of the interrupter when the interrupter is operated to disconnect a capacitive load from an electric power system. For example, the electric power system may carry an AC voltage defining a maximum voltage of about one per-unit, the capacitive load may be charged to about one per-unit, and the voltage-level threshold for the voltage-clamping device may be selected to be about one per-unit. In this configuration, the parallel-connected voltage-clamping device may operate to discharge the capacitive load while limiting the voltage across the circuit interrupter to the voltage-level threshold, about one per-unit. Thus, the parallel-connected voltage-clamping device prevents restrikes from occurring within the circuit interrupter during capacitor switching, when the voltage across the interrupter would approach two per-unit if the voltage-clamping device was not present, and the two per-unit voltage level exceeds the no-restrike design voltage of the interrupter.
That the invention improves over the drawbacks of prior circuit interrupters and accomplishes the advantages described above will become apparent from the following detailed description of specific embodiments and the appended drawings and claims.